In a reciprocating machine such as an internal combustion engine, an inertial force is generated by the movement of a reciprocal movement portion such as a piston. Conventionally, in a reciprocating machine, a rotary movement portion such as a crankshaft and a balancer shaft are given an unbalance so as to generate a centrifugal force that is in balance with the inertial force of the reciprocal movement portion, thereby controlling vibration.
FIG. 11 is a view schematically showing a conventional reciprocating engine including a so-called two-axis primary balancer. The engine includes a reciprocal movement portion 570 and a rotary movement portion 530, wherein the reciprocal movement portion 570 includes a piston 550 reciprocating in a cylinder (not shown), a piston pin 555, and a conrod small end portion 560 connected to the piston 550 by the piston pin 555, and the rotary movement portion 530 includes a rotatable crankshaft 500 and a conrod big end portion 565 connected to a crank pin portion 556 of the crankshaft 500. The engine also includes a first balancer shaft 510 arranged forward of the crankshaft 500, and a second balancer shaft 520 arranged rearward of the crankshaft 500. The first balancer shaft 510 and the second balancer shaft 520 are configured to rotate at the same rotation speed as that of the crankshaft 500 and in the opposite rotation direction to that of the crankshaft 500. An axis 500c of the crankshaft 500, an axis 510c of the first balancer shaft 510 and an axis 520c of the second balancer shaft 520 are arranged on the same plane P.
Now, assume that the axis 500c of the crankshaft 500 is the origin, the axis extending from the origin toward the piston 550 along the center line of the cylinder is the Y axis and the axis perpendicular to the Y axis is the X axis as seen from the axial direction of the crankshaft 500, and the rotation direction of the crankshaft 500 is the forward rotation direction and the opposite direction to the rotation direction of the crankshaft 500 is the reverse rotation direction as seen from the axial direction of the crankshaft 500, and assume that:
LxB(Fr): X coordinate value of the axis 510c of the first balancer shaft 510
LyB(Fr): Y coordinate value of the axis 510c of the first balancer shaft 510
LxB(Rr): X coordinate value of the axis 520c of the second balancer shaft 520
LyB(Rr): Y coordinate value of the axis 520c of the second balancer shaft 520
γB(Fr): angle, in the forward rotation direction, of the straight line connecting between the axis 510c of the first balancer shaft 510 and the axis 500c of the crankshaft 500, from the Y axis, as seen from the axial direction of the crankshaft 500=90°+arctan(LyB(Fr)/LxB(Fr))
γB(Rr): angle, in the forward rotation direction, of the straight line connecting between the axis 520c of the second balancer shaft 520 and the axis 500c of the crankshaft 500, from the Y axis, as seen from the axial direction of the crankshaft 500 (=γB(Fr))
θCr: rotation angle of the crankshaft 500 in the forward rotation direction (where θCr is assumed to be 0° when the crank pin portion 556 of the crankshaft 500 is on the Y axis)
UP: unbalance amount of the reciprocal movement portion 570=mP×R (where mP is the mass of the reciprocal movement portion 570, and R is the crank radius)
UCr: unbalance amount of the rotary movement portion 530=mCr×rCr (where mCr is the mass of the rotary movement portion 530, and rCr is the distance between the axis 500c of the crankshaft 500 and the center of gravity of the rotary movement portion 530)
αCr: unbalance direction (the angle in the forward rotation direction from the Y axis) of the rotary movement portion 530 when θCr=0°
UB(Fr): unbalance amount of the first balancer shaft 510=mB(Fr)×rB(Fr) (where mB(Fr) is the mass of the first balancer shaft 510, and rB(Fr) is the distance between the axis 510c of the first balancer shaft 510 and the center of gravity of the first balancer shaft 510)
αB(Fr): unbalance direction (the angle in the forward rotation direction from the Y axis) of the first balancer shaft 510 when θCr=0°
UB(Rr): unbalance amount of the second balancer shaft 520=mB(Rr)×rB(Rr) (where mB(Rr) is the mass of the second balancer shaft 520, and rB(Rr) is the distance between the axis 520c of the second balancer shaft 520 and the center of gravity of the second balancer shaft 520)
αB(Rr): unbalance direction (the angle in the forward rotation direction from the Y axis) of the second balancer shaft 520 when θCr=0°
In a conventional method for designing a reciprocating machine, UCr, αCr, UB(Fr), UB(Rr), αB(Fr) and αB(Rr) are set so as to satisfy setting formulae below:UCr=UP×0.5αCr=180°UB(Fr)=UP×0.5×(LxB(Rr)/sin γB(Rr))/{(LxB(Rr)/sin γB(Rr))−(LxB(Fr)/sin γB(Fr))}αB(Fr)=180°UB(Rr)=UP×0.5×(LxB(Fr)/sin γB(Fr))/{(LxB(Fr)/sin γB(Fr))−(LxB(Rr)/sin γB(Rr))}αB(Rr)=180°.
According to the reciprocating machine and the design method described above, the primary inertial force of the reciprocal movement portion 570, the centrifugal force generated by the rotary movement portion 530, the centrifugal force generated by the first balancer shaft 510 and the centrifugal force generated by the second balancer shaft 520, which are generated by the operation of the reciprocating machine, are in balance with each other. Moreover, the moments generated due to difference between lines of action of the primary inertial force and the centrifugal forces are in balance with each other. Thus, it is possible to highly control the translational vibration caused by the primary inertial force and the centrifugal forces of the reciprocating machine, and the rotational vibration caused by the moments.
With the limitation on the layout of the reciprocating machine, etc., it is difficult in some cases to arrange the axis 500c of the crankshaft 500, the axis 510c of the first balancer shaft 510 and the axis 520c of the second balancer shaft 520 on the same plane P. For example, Patent Document No. 1 discloses an engine, in which the axis of the crankshaft, the axis of the first balancer shaft and the axis of the second balancer shaft are not arranged on the same plane.